The frequency splitting between the dip and the peak of the resistively detected nuclear magnetic resonance dispersive line shape has been measured in the quantum Hall effect regime as a function of filling factor, carrier density, and nuclear isotope. The splitting increases as the filling factor tends to $\nu =1$ and is proportional to the hyperfine coupling, similar to the usual Knight shift versus $\nu$ dependence. The peak frequency shifts linearly with magnetic field throughout the studied filling factor range and matches the unshifted substrate signal, detected by classical NMR. Thus, the evolution of the splitting is entirely due to the changing Knight shift of the dip feature. The nuclear spin relaxation time $T_1$ is extremely long ($\sim$hours) at precisely the peak frequency. These results are consistent with the local formation of a $\nu =2$ phase due to the existence of spin singlet $D^{-}$ complexes.

We study the phase diagrams of a family of three-dimensional Walker-Wang–type lattice models, which are not topologically ordered but have deconfined anyonic excitations confined to their surfaces. We add a perturbation (analogous to that which drives the confining transition in ${\mathbb{Z}}_p$ lattice gauge theories) to the Walker-Wang Hamiltonians, driving a transition in which all or some of the variables associated with the loop-gas or string-net ground states of these models become confined. We show that in many cases the location and nature of the phase transitions involved is exactly that of a generalized ${\mathbb{Z}}_p$ lattice gauge theory, and use this to deduce the basic structure of the phase diagram. We further show that the relationship between the phases on opposite sides of the transition is fundamentally different than in conventional gauge theories: in the Walker-Wang case, the number of species of excitations that are deconfined in the bulk can increase across a transition that confines only some of the species of loops or string nets. The analog of the confining transition in the Walker-Wang models can therefore lead to bulk deconfinement and topological order.

We apply diffusion quantum Monte Carlo to a broad set of solids, benchmarking the method by comparing bulk structural properties (equilibrium volume and bulk modulus) to experiment and density functional theory (DFT) based theories. The test set includes materials with many different types of binding including ionic, metallic, covalent, and van der Waals. We show that, on average, the accuracy is comparable to or better than that of DFT when using the new generation of functionals, including one hybrid functional and two dispersion corrected functionals. The excellent performance of quantum Monte Carlo on solids is promising for its application to heterogeneous systems and high-pressure/high-density conditions. Important to the results here is the application of a consistent procedure with regards to the several approximations that are made, such as finite-size corrections and pseudopotential approximations. This test set allows for any improvements in these methods to be judged in a systematic way.

We present measurements of the microwave surface impedance of the single-layer cuprate Tl${}_2$Ba${}_2$CuO${}_{6+\delta }$, deep in the overdoped regime, with $T_c\approx 25$ K. Measurements have been made using cavity perturbation of a dielectric resonator at 17 discrete frequencies ranging from 2.50 to 19.16 GHz, and at temperatures from 0.12 to 27.5 K. From the surface impedance we obtain the microwave conductivity, penetration depth, and superfluid density. The superfluid density displays a strong linear temperature dependence from 2 to 14 K, indicative of line nodes in the energy gap. The microwave data are compared with theoretical predictions for a $d$-wave superconductor with pointlike impurities, with the conclusion that disorder in Tl${}_2$Ba${}_2$CuO${}_{6+\delta }$ acts predominantly in the weak-to-intermediate-strength scattering regime, and that small-angle scattering is important.

The semiclassical motion of electrons in phase space $\mathbf{x}=(\mathbf{R},\mathbf{k})$ is influenced by Berry phases described by a six-component vector potential $\mathbf{A}=({\mathbf{A}}^{\mathrm{R}},{\mathbf{A}}^{\mathrm{k}})$. In chiral magnets, Dzyaloshinskii-Moriya (DM) interactions induce slowly varying magnetic textures (helices and skyrmion lattices) for which all components of $\mathbf{A}$ are important, inducing effectively a curvature in mixed position and momentum space. We show that for smooth textures and weak spin-orbit coupling, phase-space Berry curvatures determine the DM interactions and give important contributions to the charge. Using ab initio methods, we calculate the strength of DM interactions in MnSi in good agreement with experiment and estimate the charge of skyrmions.

We present experimental and theoretical studies of the magneto-optical properties of $p$-type In${}_{1-x}$Mn${}_x$As and In${}_{1-x}$Mn${}_x$Sb ferromagnetic semiconductor films in ultrahigh magnetic fields oriented along [001]. Samples were fabricated by molecular beam epitaxy (MBE) and metal-organic vapor phase epitaxy (MOVPE). To model the results, we used an 8-band Pidgeon-Brown model generalized to include the wave vector dependence of the elec-tronic states along $k_z$ as well as the s-d and p-d exchange interactions with the localized Mn $d$ electrons. The Curie temperature is taken as an input parameter and the average Mn spin is treated in mean-field theory. We compared Landau level and band structure calculations with observed cyclotron resonance (CR) measurements. While differences between the CR measurements are seen for MBE and MOVPE samples, our calculations indicate that they arise from differences in the carrier densities. In addition, the difference in the carrier densities suggests significantly larger average spin for the MOVPE structures; this fact could be responsible for higher Curie temperatures in this material system.

We study a Hanbury Brown and Twiss (HBT) interferometer formed with chiral edge channels of a quantum Hall system. HBT cross correlations are calculated for a device operating both in the integer and fractional quantum Hall regimes, the latter at Laughlin filling fractions. We find that in both cases, when the current is dominated by electron tunneling, current-current correlations show antibunching, characteristic of fermionic correlations. When the current-current correlations are dominated by quasiparticle tunneling, the correlations reveal bunching, characteristic of bosons. For electron tunneling, we use the Keldysh technique, and show that the result for fractional filling factors can be obtained in a simple way from the results of the integer case. It is shown that quasiparticle-dominated cross-current correlations can be analyzed by means of a quantum master-equation approach. We present here a detailed derivation of the results [Campagnano et al., Phys. Rev. Lett. 109, 106802 (2012)] and generalize them to all Laughlin fractions.

We report an experimental investigation of the fractional quantum Hall effect (FQHE) at the even-denominator Landau-level filling factor $\nu =1/2$ in very-high-quality wide GaAs quantum wells and at very high magnetic fields up to 45 T. The quasi-two-dimensional electron systems we study are confined to GaAs quantum wells with widths $W$ ranging from 41 to 96 nm and have variable densities in the range of $\simeq$$4\times {10}^{11}$ to $\simeq$$4\times {10}^{10}$ cm${}^{-2}$. We present several experimental phase diagrams for the stability of the $\nu =1/2$ FQHE in these quantum wells. In general, for a given $W$, the 1/2 FQHE is stable in a limited range of intermediate densities where it has a bilayerlike charge distribution; it makes a transition to a compressible phase at low densities and to an insulating phase at high densities. The densities at which the $\nu =1/2$ FQHE is stable are larger for narrower quantum wells. Moreover, even a slight charge distribution asymmetry destabilizes the $\nu =1/2$ FQHE and turns the electron system into a compressible state. We also present a plot of the symmetric-to-antisymmetric subband separation (${\Delta }_{\mathrm{SAS}}$), which characterizes the interlayer tunneling, vs density for various $W$. This plot reveals that ${\Delta }_{\mathrm{SAS}}$ at the boundary between the compressible and FQHE phases increases linearly with density for all the samples. There is no theoretical explanation for such a simple dependence. Finally, we summarize the experimental data in a diagram that takes into account the relative strengths of the interlayer and intralayer Coulomb interactions and ${\Delta }_{\mathrm{SAS}}$. We conclude that consistent with the conclusions of some of the previous studies, the $\nu =1/2$ FQHE observed in wide GaAs quantum wells with symmetric charge distribution is stabilized by a delicate balance between the interlayer and intralayer interactions and is very likely described by a two-component (${\Psi }_{331}$) state.

The ultimate miniaturization of electronic devices at the atomic scale with single electrons requires controlling the reversible charge storage in a single atom. However, reversible charge storage is difficult to control as usually only one charge state can be stabilized. Here, combining scanning tunneling microscopy (STM) and density functional theory (DFT), we demonstrate that a single silicon dangling bond of a hydrogenated $p$-type doped Si(100) surface has two stable charge states (neutral and negatively charged) at low temperature (5 K). Reversible charge storage is achieved using a gate electric field between the STM tip and the surface.

Using high-resolution angle-resolved photoemission spectroscopy, we studied the electronic structure of NaFe${}_{1-x}$Cu${}_x$As ($x=0.019,0.045,0.14$). With increasing the doping concentration, we found that the Cu dopant introduces extra charge carriers. The overall band dispersions barely change with doping, suggesting that the Cu substitution does not affect local correlations. Similar to the case of NaFe${}_{1-x}$Co${}_x$As, one electron pocket emerges at the Brillouin zone center at high doping levels. Moreover, the near-E${}_F$ spectral weight decreases with increasing the Cu dopant, which explains why the NaFe${}_{1-x}$Cu${}_x$As shows a poor electronical conductivity at high doping levels.

Relativistic O($N$) field theories are studied near the quantum-critical point in two space dimensions. We compute dynamical correlations by large-scale Monte Carlo simulations and numerical analytic continuation. In the ordered side, the scalar spectral function exhibits a universal peak at the Higgs mass. For $N=3$ and 4, we confirm its ${\omega }^3$ rise at low frequency. On the disordered side, the spectral function exhibits a sharp gap. For $N=2$, the dynamical conductivity rises above a threshold at the Higgs mass (density gap), in the superfluid (Mott insulator) phase. For charged bosons (Josephson arrays), the power-law rise above the Higgs mass increases from two to four. Approximate charge-vortex duality is reflected in the ratio of imaginary conductivities on either side of the transition. We determine the critical conductivity to be ${\sigma }_{\mathrm{c}}^{*}=0.3(\pm 0.1)\times 4e^2/h$ and describe a generalization of the worm algorithm to $N>2$. We use a singular value decomposition error analysis for the numerical analytic continuation.

Nanostripes with very high (>20) aspect ratios of a two-dimensional manganese oxide phase are grown on the Ag(100) surface using a physical vapor deposition method and thoroughly characterized via low-energy electron diffraction and scanning tunneling microscopy. Despite the square pattern of the substrate, symmetry breaking is produced by the structural anisotropy of the kinetically stabilized, metastable Mn surface oxide phase. As rationalized by a first-principles analysis, the resulting asymmetry in edge diffusion and attachment energies and the difference in adsorption energetics between Mn and Ag adatoms leads to the kinetic formation of such strikingly anisotropic structural domains: they feature nanoscale width (≈5–20 nm), mesoscopic length (up to ≈1 μ), very low defect density, and grow along the high-symmetry directions of the Ag(100) substrate.

From a direct calculation of the anomalous Hall conductivity and an effective electromagnetic action obtained via Fujikawa's chiral rotation technique, we conclude that an axionic field theory with a nonquantized coefficient describes the electromagnetic response of the $(3+1)$-dimensional Weyl semimetal. The coefficient is proportional to the momentum space separation of the Weyl nodes. Akin to the Chern-Simons field theory of quantum Hall effect, the axion field theory violates gauge invariance in the presence of the boundary, which is cured by the chiral anomaly of the surface states via the Callan-Harvey mechanism. This provides a unique solution for the radiatively induced CPT-odd term in the electromagnetic polarization tensor of the Lorentz violating spinor electrodynamics, where the source of the Lorentz violation is a constant axial 4-vector term for the Dirac fermion. A direct linear response calculation also establishes anomalous thermal Hall effect and a Wiedemann-Franz law, but thermal Hall conductivity does not directly follow from the well known formula for the gravitational chiral anomaly.

We study by high-resolution transmission electron microscopy the structural response of bilayer graphene to electron irradiation with energies below the knock-on damage threshold of graphene. We observe that one type of divacancy, which we refer to as the butterfly defect, is formed for radiation energies and doses for which no vacancies are formed in clean monolayer graphene. By using first principles calculations based on density-functional theory, we analyze two possible causes related with the presence of a second layer that could explain the observed phenomenon: an increase of the defect stability or a catalytic effect during its creation. For the former, the obtained formation energies of the defect in monolayer and bilayer systems show that the change in stability is negligible. For the latter, ab initio molecular dynamics simulations indicate that the threshold energy for direct expulsion does not decrease in bilayer graphene as compared with monolayer graphene, and we demonstrate the possibility of creating divacancies through catalyzed intermediate states below this threshold energy. The estimated cross section agrees with what is observed experimentally. Therefore, we show the possibility of a catalytic pathway for creating vacancies under electron radiation below the expulsion threshold energy.

We study the conductance through an Aharonov-Bohm ring, containing a quantum dot in the Kondo regime in one arm, at finite temperature and arbitrary electronic density. We develop a general method for this calculation based on changing the basis to the screening and nonscreening channels. We show that an unusual term appears in the conductance, involving the connected four-point Green's function of the conduction electrons. However, this term and the terms quadratic in the $\mathrm{T}$ matrix can be eliminated at sufficiently low temperatures, leading to an expression for the conductance linear in the Kondo T matrix. Explicit results are given for temperatures that are high compared to the Kondo temperature.

We theoretically investigate the ground-state properties of a molecular para-hydrogen (p-H${}_2$) film in which crystallization is energetically frustrated by embedding sodium (Na) atoms periodically distributed in a triangular lattice. In order to fully deal with the quantum nature of p-H${}_2$ molecules, we employ the diffusion Monte Carlo method and realistic semiempirical pairwise potentials describing the interactions between H${}_2$-H${}_2$ and Na-H${}_2$ species. In particular, we calculate the energetic, structural, and superfluid properties of two-dimensional Na-H${}_2$ systems within a narrow density interval around equilibrium at zero temperature. In contrast to previous computational studies considering other alkali metal species such as rubidium and potassium, we find that the p-H${}_2$ ground state is a liquid with a significantly large superfluid fraction of ${\rho }_s/\rho =0.29\left(2\right)$. The appearance of p-H${}_2$ superfluid response is due to the fact that the interactions between Na atoms and H${}_2$ molecules are less attractive than between H${}_2$ molecules. This induces a considerable reduction of the hydrogen density which favors the stabilization of the liquid phase.

Plasmons in two-dimensional electron systems with nonparabolic bands, such as graphene, feature strong dependence on electron-electron interactions. We use a many-body approach to relate plasmon dispersion at long wavelengths to Landau Fermi-liquid interactions and quasiparticle velocity. An identical renormalization is shown to arise for the magnetoplasmon resonance. For a model with $N\gg 1$ fermion species, this approach predicts a power-law dependence for plasmon frequency vs carrier concentration, valid in a wide range of doping densities, both high and low. Gate tunability of plasmons in graphene can be exploited to directly probe the effects of electron-electron interaction.

We derive the gradient expansion for the exchange energy of a spin-polarized electron gas by perturbing the uniformly spin-polarized state and thus inducing a small noncollinearity that is slowly varying in space. We show that the contribution due to the induced longitudinal gradient of the spin polarization to the exchange energy differs from the contribution due to the transverse gradient. The difference is present at any nonvanishing spin polarization and becomes larger with increasing spin polarization. We argue that improved generalized gradient approximations of spin-density-functional theory must account for the difference between the longitudinal- and transverse-spin stiffness.

The double-differential spectrum of coincidences between backscattered electrons and secondary electrons (SEs) emitted from a polycrystalline Al surface bombarded with 100-eV electrons was measured. For energy losses of the scattered electron in between the work function of Al and the bulk plasmon energy, a sharp peak is observed in the SE spectra, corresponding to ejection of a single electron near the Fermi edge receiving the full energy loss and momentum of the primary electron. This process predominantly takes place when the primary electron suffers a surface energy loss in vacuum, and leads to SE ejection from the very surface. At energy losses just above the bulk plasmon energy, a sharp transition is observed, corresponding to a sudden increase in the depth of ejection. The latter is a direct consequence of the complementarity of surface and bulk plasmons, the so-called Begrenzungs effect.

A novel experimental technique, resonant piezoelectric spectroscopy (RPS), has been applied to investigate polar precursor effects in highly (65$\%$) B-site ordered PbSc${}_{0.5}$Ta${}_{0.5}$O${}_3$ (PST), which undergoes a ferroelectric phase transition near 300 K. The cubic-rhombohedral transition is weakly first order, with a coexistence interval of $\sim$4 K, and is accompanied by a significant elastic anomaly over a wide temperature interval. Precursor polarity in the cubic phase was detected as elastic vibrations generated by local piezoelectric excitations in the frequency range 250–710 kHz. The RPS resonance frequencies follow exactly the frequencies of elastic resonances generated by conventional resonant ultrasound spectroscopy (RUS) but RPS signals disappear on heating beyond an onset temperature, $T_{\mathrm{onset}}$, of 425 K. Differences between the RPS and RUS responses can be understood if the PST structure in the precursor regime between $T_{\mathrm{onset}}$ and the transition point, $T_{\mathrm{trans}}=300$ K, has locally polar symmetry even while it remains macroscopically cubic. It is proposed that this precursor behavior could involve the development of a tweed microstructure arising by coupling between strain and multiple order parameters, which can be understood from the perspective of Landau theory. As a function of temperature the transition is driven by the polar displacement $P$ and the order parameter for cation ordering on the crystallographic B site $Q_{\mathrm{od}}$. Results in the literature show that, as a function of pressure, there is a separate instability driven by octahedral tilting for which the assigned order parameter is $Q$. The two mainly displacive order parameters, $P$ and $Q$, are unfavorably coupled via a biquadratic term $Q^2{P}^2$, and complex tweedlike fluctuations in the precursor regime would be expected to combine aspects of all the order parameters. This would be different from the development of polar nanoregions, which are more usually evoked to explain relaxor ferroelectric behavior, such as occurs in PST with a lower degree of B-site order.